Heat exchanger with improved plates

ABSTRACT

Disclosed is a heat exchanger composed of corrugated plates, the facets (12) of which are provided with bosses (13) and hollows (14) in order to reduce pressure drops.

DESCRIPTION

This invention relates to a plate-type heat exchanger made up of a stackof corrugated plates delimiting channels of variable section.

Corrugations have the general purpose of disturbing the flow of fluidsto increase the heat transfers through the plates, but they have thedrawback of making the pressure drops much larger than with flat plates.An earlier French patent (FR-A-2 648 220) described a particular form ofcorrugated plate which made it possible to reduce the volume of dead orrecirculation zones in which the fluid stagnates substantially, which isone of the main causes of losses in heat exchange efficiency and thefouling of plates if the fluid is charged with particles, because theythen deposit much more easily. The plates of that earlier invention hadfacets of two alternately different lengths and were assembled togethersuch that the corrugations of the consecutive plates formed angles ofpreference of nearly 180° C., i.e. such that, in each channel, the longfacets were oriented substantially in the same direction correspondingto the direction of fluid flow in the channel: the short facets thusfaced the fluid and, owing to their steeper slope, deviated it stronglytowards the long facets of the other plate delimiting the channel. Theresult was that the fluid stream licked the long facets over a largepart of their surface, and the recirculation zones which formed behindthe corrugations which restricted the channel, namely in front of thelong facets, were reduced as a consequence. One thus obtained betterheat exchange performance as well as easier flow.

The objectives assigned to that invention were thus met, but certaindrawbacks may be found to such a configuration of plates. First of all,the corrugations composed of a long facet and a short facet have, forthe same pitch (the pitch designating the width of the corrugation), asmaller height than the corrugations of a conventional corrugated platecomposed of two similar facets. In other words, the channels have asmaller average section. In practice, it is however desirable not toreduce this section, thus requiring the use of plates of similar shapebut with larger corrugations having a larger pitch. The lines ofcorrugations then have a larger spacing and the number of contact pointsbetween the plates is smaller, thus reducing the mechanical strength ofthe stack.

Another drawback stems from the fact that the beneficial effect isobtained only for one direction of fluid flow in each channel,alternating in adjacent channels, thus imposing countercurrentcirculation of fluids which is not always desired. Finally, if pressuredrops are lower than with conventional corrugated plates, they remainsignificant.

The present invention may be regarded as an improvement of the earlierinvention, because it offers substantially equivalent advantages asconcerns the heat exchange performance and the reduced fouling of thewalls of the plates, but also substantially smaller pressure drops bythe reduction of vortex movements, while still involving the same numberof contact points between plates as ordinary corrugated plates. In itsbest embodiments, the invention also lends itself to similar flows offluids in both directions, thus allowing the free choice of parallelcurrent flows as well as counterflows.

The invention relates, in its most general form, to a heat exchangercomposed of corrugated plates placed side by side to delimit channels,the plates being similar, composed of facets joined by bottom and toplines, with the plates joined to each other at contact points,characterized in that the plates are alternately turned over and joinedeither through their top lines or through their bottom lines, and inthat the facets include bosses near the top lines and hollows near thebottom lines.

This thus provides an arrangement in which the corrugations areflattened and squeeze the section of the channels near the contactpoints of the corrugations, thanks to this combination of hollows andbosses as described and to the alternate turnover of the plates. TheU.S. Pat. No. 4,014,385 describes an arrangement in which the facetsinclude, on the contrary, hollows at the top and bosses at the bottom,so that they are connected to each other to form right angles, whichallows the stiffening of the plates but has an effect opposite to thatof the invention on the flow of fluid.

It is recommended that hollows and bosses should be discontinuous alongthe facets, with the bosses located near the contact points, thusenabling them to play the channel volume reduction role only at thelocations in which dead or stagnant zones are most likely to form. Thebosses are much less useful elsewhere. A simple construction is that inwhich the facets include hollows and bosses which alternate. Finally, ifeach contact point is located between two bosses belonging to adjacentfacets of the same plate, the reversibility of the flow in the channelsis assured.

The invention will now be described in greater detail in conjunctionwith the following figures appended by way of non-limitativeillustration:

FIG. 1 is a general view of a plate-type heat exchanger in which theplates are represented in an exploded view for clarity;

FIG. 2 is a partial sectional view of a plate according to theinvention;

FIG. 3 is a top view of a plate of the invention, showing thedistribution of the bosses and hollows;

FIG. 4 illustrates how the plates are superposed and in particular theangle of their corrugations; and

FIG. 5 illustrates the respective positions of the hollows and bosses onthe two superposed plates, according to a plane section indicated by theline V--V on a plate in FIG. 3.

A plate-type heat exchanger of a current type is shown in FIG. 1. It ismade up of a superposition of rectangular plates 1, having four bores 2in the corners, a smooth peripheral groove 3 and corrugations 4 on therest of their surface. The plates may be produced by various means, bystamping, machining or casting, and in the real heat exchanger they bearon each other through their corrugations 4. Gaskets, not represented,are then compressed between the grooves 3 and provide leaktightness. Thestack is held by clamping.

In the represented embodiment, there is a countercurrent fluid flow, butit could be otherwise. The corrugations 4 are of herringbone form butcould be straight. The fluids are generally liquids in existingembodiments, but this is not obligatory, and there may also be changesof state. The invention is applicable to all these categories ofexchangers and even to different kinds of exchangers.

According to the invention (FIGS. 2 to 5), the corrugations 4 of theplates 1 may be broken down into top lines 10 alternating with bottomlines 11, the lines 10 and 11 all being parallel to each other andseparating contiguous facets 12. The facets 12 have a rough surface,i.e. they are not straight over the essential part of their length as inconventional corrugated plates, but exhibit bosses 13 and hollows 14.For the requirements of the description, the observation reference islocated in the channel 6 over the plate 1, and the top lines 10 are overthe bottom lines 11; the bosses 13 are convex reliefs and the hollows 14are concave reliefs in this channel 6.

Hollows and bosses 14 and 13 are produced without difficulty with thecorrugations 4, for example by stamping with special dies, without anyspecial operation. The general representation in FIG. 3 shows that thehollows 14 and the bosses 13 do not extend over the entire length of thecorrugations 4 but are, on the contrary, discontinuous and that thebosses 13 extend near the top lines 10, approximately over half of thelength of the corrugations 4, and to the locations near the contactpoints 15 of the adjacent plate 1; more precisely, the bosses 13 of theadjacent facets 12 extend on either side of the middle top line 10 so asto surround in pairs the contact points 15. The hollows 14 are adjacentto the bottom lines 11 and each extends between two consecutive bosses13 of the facet 12 to which they belong (hollow 14 and bosses 13 thusalternate along each of the facets 12), so that they form a roughlyuninterrupted series along each of the bottom lines 11, alternately onthe two facets 12 which border it. In this embodiment, the averagedirection of the flow of fluid in the channel 6 is vertical (accordingto the representation of this FIG. 3) and the angle α of thecorrugations 4 with this direction is 60°. The other plate 1 delimitingthe channel 6 will be similar but placed after having been tuned over,so that the plates 1 will join through their top lines 10 (FIGS. 4 and5), their corrugations 4 being crossed and forming angles of 60°. Thesame will be true for each couple of plates 1, which will be joinedeither through their respective top lines 10 or through their bottomlines 11. Whether the direction of flow is ascending or descending, thecharacteristics of the flow are identical because the form of thechannels 6 is symmetrical. It is noted in particular that the bosses 13form zones in which the section of the channel 6 is very small aroundthe contact points 15, in which the fluid would have a tendency tostagnate, but that the bosses 13 do not contribute to hindering the flowat the other locations of the channels 6.

Similarly, the contact points 16 with the other adjacent plate 1,located at the center of the diamonds formed by four adjacent contactpoints 15, will be surrounded by two pairs of hollows 14 of the twoplates 1 concerned, but which are seen as bosses 13 in the adjacentchannel 6 delimited by these two plates 1. It is thus seen that all thechannels will have the same form.

The hollows 14 have hardly any influence on the flow in the channels 6.

To allow the proper positioning of the plates in relation to each other,hollows 17 may be provided on the top lines 10 at the contact points 15,between the bosses 13. These hollows make it possible, on the one hand,to position the plates precisely and, on the other, to bettor streamlinethe flow around the corrugation. These hollows 17 have a depth of about0. 5 mm (between 0.3 mm and 1 mm) and their form allows engagement ofthe contact points 15 of the upper plate. They are made in the same wayas the rest of the plate, with no additional Cost. The hollows 17 areestablished on every other plate 1; the top lines 10 of the other plates1 remain straight.

Bosses could also be provided at the bottom lines 11, at the location ofthe contact points 16, on certain plates 1 to facilitate also theengagement of the plates 1 by these bottom lines 11.

Other embodiments are possible depending in particular on the anglesformed by the corrugations in relation to the average direction of flow.

An embodiment actually tested included plates with corrugations 4 inwhich the angle α in relation to the flow was equal to 60°, the plates 1having a pitch p (FIGS. 2 and 3) of 13 mm and a height e (FIG. 2) of 3.9mm, for maximum heights and depths of 0.8 mm and diameters from 3 to 4mm for the bosses 13 and the hollows 14. The channels 6 were 0.4 m longand 0.14 m wide. The flow rate was 6 to 40 m³ per hour in each channel6. The plates 1 in accordance with the invention produced a head lose(or pressure drop) from 30 to 50% smaller than a conventional corrugatedplate, i.e. without bosses 13 and hollows 14. The heat exchangecoefficients were similar, with differences of less than 5%. The platesconsisted or stainless-steel stampings 0.6 mm thick.

The invention can be applied to all fields of activity in whichexchangers of this type are used, and in particular the chemical,para-chemical, petroleum, climatic, agro-food, energy production andmetallurgical industries.

We claim:
 1. A heat exchanger comprising stacked corrugated plates, theplates defining fluid channels therebetween, the plates being composedof facets joined at bottom lines and top lines, wherein the plates aresimilar in shape and alternatively turned over, the top lines of a firstof the plates being joined to the top lines of another of the plates andthe bottom lines of the first plate being joined to the bottom lines ofstill another one of the plates at contact points, the facets includingbosses near the top lines and hollows near the bottom lines.
 2. The heatexchanger of claim 1, wherein the hollows and the bosses arediscontinuous along the facets and the bosses are located near thecontact points.
 3. The heat exchanger of claim 2, wherein the hollowsand the bosses alternate along the facets.
 4. The heat exchanger ofclaim 2, wherein each of the contact points is located between a pair ofthe bosses.
 5. The heat exchanger of claim 4, wherein one boss of thepair of bosses is located on one of the facets and a second boss of thepair of bosses is located on a second adjacent one of the facets.
 6. Theheat exchanger of claim 1, wherein special hollows are located at thecontact points to provide a fitting adjustment of the plates.